1. Field of the Invention
The present invention relates to wireless transmission systems.
2. Description of the Related Art
Transmitted signals of a digital transmission system are usually configured to a single modulation scheme. The modulation scheme is often determined as a result of the region in which the signal is to be transmitted. For instance, time division multiplexed (TDM) signals are typically suited for rural areas whereas orthogonal frequency multiplexed (OFDM) signals are typically suited for urban areas.
TDM signals are suited for rural areas, where there is typically a clear line-of-sight (LOS) between a satellite transmitter and a ground-based receiver. Often a time-delayed signal may also be transmitted to compensate for short-term signal outages that may occur when there is an obstruction in the LOS signal path (e.g., when a mobile receiver passes under an overpass). In such cases, after the direct LOS connection has been reestablished, any data in the xe2x80x9con-timexe2x80x9d LOS signal that was lost during the temporary obstruction will be available from the time-delayed TDM signal.
In urban areas, buildings and other structures form potentially long-lasting LOS obstructions. They also tend to act as a source of reflections leading to multipath signal distortions. Consequently, TDM-based service is often unacceptable in such urban areas. Since OFDM signals are well-suited for regions, such as urban areas, having LOS obstructions and multipath signals, OFDM signals are typically used.
Since each type of signal is ideally suited for different operating conditions, it is typical to use each signal only in the region suited for each signal. Additionally, it is typical to encounter increased noise at transition regions where, for example, a mobile receiver may be changing between TDM service and OFDM service. Noise at such transition regions often causes signal degradation, handoff failures, and signal losses, which detrimentally affect the quality of the service.
The present invention is directed to a technique for improving the quality of service for wireless transmission systems that employ two or more different modulation schemes to transmit the same information in parallel, such as satellite-based digital audio transmission systems that transmit digital audio data using both TDM and OFDM modulation schemes.
In the satellite-based digital audio transmission system, two TDM signals and one OFDM signal are used. A transmitter transmits two TDM signals that carry the same information, where one signal is delayed in time (e.g., by a few seconds) in relation to the other. The two TDM signals are used to compensate for short-term signal outages that may occur when there is an obstruction in the LOS signal path (e.g., when a mobile receiver passes under an overpass). The transmitter also transmits an OFDM signal which is typically used as a terrestrial gap filler signal for regions where TDM signals do not provide acceptable service. In particular, OFDM signals carrying the same information as the TDM signals are transmitted in parallel with the TDM signals, and terrestrial OFDM repeaters are deployed to fill the regional gaps in the TDM service.
According to one embodiment of the present invention, for TDM/OFDM digital audio transmission systems, a TDM/OFDM transmitter transmits two TDM signals and a single OFDM signal that are then received at a receiver. The two TDM signals (i.e., the on-time TDM signal and the time-delayed TDM signals) and the single OFDM signal received at a mobile receiver are combined using suitable signal combining techniques to generate a single combined signal for subsequent signal processing (e.g., signal decoding). By combining the differently modulated signals, the adverse affects of noise in the individual signals can be reduced. As a result, the occurrence of signal losses and handoff failures may also be reduced.
In general, the principles of the present invention can be applied to improve the quality of service for any signal transmission system that uses two or more different modulation schemes to transmit the same information in parallel. Satellite-based digital audio transmission systems based on TDM and OFDM modulation schemes are just one particular application of the present invention.
In one particular implementation of the present invention for a TDM/OFDM digital audio transmission system, the two TDM signals (i.e., the on-time TDM signal and the time-delayed TDM signals) and the single OFDM signal are received at a receiver, where they are demodulated and combined using a maximal ratio combining (MRC) technique to generate a combined signal for further processing (e.g., decoding). The combining of the two differently-modulated signals reduces the adverse effects of noise, thereby improving quality of service.
In one embodiment, the present invention is a method for processing wireless signals, comprising the steps of (a) receiving two or more wireless signals containing a common set of information and conforming to two or more different modulation schemes; (b) demodulating each of the received wireless signals using a corresponding different demodulation scheme; and (c) combining the two or more demodulated signals using a signal combining technique to generate a combined signal.
In another embodiment, the present invention is an apparatus for processing wireless signals, comprising (a) an antenna, configured to receive an analog signal corresponding to two or more wireless signals containing a common set of information and conforming to two or more different modulation schemes; (b) a converter, electrically connected to the antenna and configured to convert the analog signal to a baseband signal; (c) a separator, electrically connected to the converter and configured to separate the baseband signal into two or more sub-signals corresponding to the two or more wireless signals; (d) for each sub-signal, a demodulator electrically connected to the separator and configured to apply demodulation processing corresponding to the modulation scheme for the corresponding sub-signal; (e) a synchronizer, electrically connected to each demodulator and configured to synchronize each demodulated sub-signal; and (f) a signal combiner, electrically connected to the synchronizer and configured to combine the two or more demodulated sub-signals using a signal combining technique to generate a combined digital signal.